The present invention relates to a method for producing a silver salt of an organic acid and a photothermographic material utilizing the silver salt.
In recent years, reduction of amount of waste processing solutions is strongly desired in the medical field from standpoints of environmental protection and space savings. Techniques relating to photosensitive thermographic materials for use in the medical field and photographic-art processes are required which enables efficient exposure by a laser image setter or a laser imager and formation of a clear black image having high resolution and sharpness. The photosensitive thermographic materials can provide users with a more simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
The same need applies to the field of ordinary image-forming materials. However, photo-images for medical use require high quality excellent in sharpness and graininess as they need very fine images. In addition, for easy diagnosis, cold monochromatic images are preferred. At present, various types of hard copy systems using pigments and dyes, for example, ink jet printers and electrophotographic systems are available as ordinary image forming systems. However, no satisfactory system is available for medical use.
Methods utilizing a silver salt of an organic acid for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and Klostervoer, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Generally, the phototlhermographic material, in particular, comprises a image-forming layer (photosensitive layer) containing a photocatalyst (e.g., silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (e.g., silver salt of an organic acid), and optionally a toning agent for controlling tone of silver, which are usually dispersed in a binder matrix. When the photothermographic material is heated at a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, a monochromatic black silver image is produced through an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of silver halide generated upon exposure. Therefore, the monochromatic silver images are formed in exposed areas of the materials. This technique is disclosed in many references including U.S. Pat. No. 2,910,377 and Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) 43-4924. The photothermographic systems using a silver salt of an organic acid can achieve image quality and tones that satisfy the needs in the medical filed.
The silver source used in these systems is generally a silver salt of a fatty acid, and various methods for producing the same are known. Examples of the methods include a method of preparing a silver salt of an organic acid under coexistence of water and a hardly water-soluble solvent as disclosed in Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-93310, JP-A-49-94619 and JP-A-53-68702, a method of preparing a silver salt of an organic acid in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709, a method of preparing a silver salt of an organic acid in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458 and so forth. Basically, the preparation is carried out by heating a fatty acid to a temperature higher than melting point thereof to dissolve the acid in water, adding sodium hydroxide or an alkali metal salt with vigorous stirring, and then adding silver nitrate to convert the alkali soap into silver soap.
Such alkali soap forms micelles in an aqueous solution, and gives a solution of whitely turbid appearance. The reaction from such a micelle state to the silver soap often suffers from problems concerning production stability. Therefore, as a method for obtaining the alkali soap as a uniform solution, a method of using a mixed solution of water and alcohol as the solvent is disclosed in JP-A-55-40607.
Further, alkali soap presents alkalinity as indicated by its name. Therefore, the silver soap will be prepared under a high pH condition in the above method. However, addition of a solution containing silver ions into an alkaline solution not only produces silver oxide as a byproduct, but generates unintended silver nuclei produced by a trace amount of reducing contaminants, which are unavoidable in a production process and exhibit high reducing property due to the high pH. Such byproducts are highly disadvantageous since they degrade performance of photothermographic materials, in particular, cause undesired fog. The problem of fog is not solved even by the method disclosed in JP-A-55-40607, which aims at obtaining a uniform solution to suppress generation of the byproducts.
In addition, JP-A-9-127643 discloses a method for producing a silver salt by simultaneous addition of measured amounts of an alkali metal salt solution and a silver nitrate solution, and the reference describes simultaneous addition of a solution of sodium behenate in a mixture of water and isopropyl alcohol and a solution of silver nitrate. This method can at least lower the high pH of the reaction to a neutral region, and thus is preferred to reduce the generation amount of silver oxide. However, isopropyl alcohol has weak reducing property, which makes the method insufficient to completely solve the problem of fog.
Moreover, the silver behenate grains formed by this method are two-dimensionally and anisotropically grown acicular grains having a size of 0.04 xcexcm to 0.05 xcexcm, and the reference contains no description concerning control of the grain size or grain shape.
In order to obtain a uniform dispersion practically usable as a coating solution containing a silver salt of an organic acid, it is necessary to achieve a state in which the silver salt of an organic acid is finely dispersed in a solvent without aggregation. For this reason, it is necessary to develop a method for dispersing the silver salt of an organic acid as fine grains. An ordinarily used method includes a method comprising the steps of separating the formed hydrophobic grains of silver salt of an organic acid as solid by filtration, mixing a dispersing agent with the solid and re-dispersing the mixture as described by Kloosterboer (Imaging Processes and Materials, Noblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989)).
As the method for dispersing a silver salt of an organic acid as fine grains, a method of mechanically dispersing the salt in the presence of a dispersing aid by a known pulverization means (e.g., high-speed mixer, homogenizer, high-speed impact mill, Banbary mixer, homomixer, kneader, ball mill, vibrating ball mill, planetary ball mill, attriter, sand mill, bead mill, colloid mill, jet mill, roller mill, trone mill and high-speed stone mill). However, these methods only produce a coating solution containing a lot of aggregated particles, i.e., a coating solution that gives bad coated surface quality, as well as they cause a, problem that, because the methods possibly grind primary grains of a silver salt of an organic salt originally crystallized as a hardly wafer-soluble salt without any selectivity, silver nuclei are formed at crystal cleavage surfaces and causes increase of fog.
Several methods have been proposed, wherein the primary grains obtained during the reaction of a solution of alkali metal salt and a solution containing silver ions are utilized as they are, not separating the silver salt of an organic acid as solid and finely dispersing the solid.
For example, JP-A-8-234358 discloses a method of adding silver nitrate to an aqueous dispersion in which fine grains of an alkali salt of an organic acid are dispersed, and desalting the obtained dispersion of a silver salt of an organic acid by ultrafiltration. This method further utilizes means for enhancing dispersion stability by carrying out the ultrafiltration by adding beforehand with water-soluble protective colloids such as polyvinyl alcohol and gelatin.
However, the shape of the silver salt of an organic acid obtained by this method is limited to an acicular shape, and in addition, it is difficult to control the grain size. Therefore, the method is still insufficient to stably achieve performance of low fog, high blackening density and low haze, which are desired for photothermographic materials.
JP-A-9-127643 discloses a method of directly desalting dispersion of silver salt of an organic acid obtained by simultaneous addition of measured amounts of solution of an alkali-metal salt and a silver nitrate solution by means of dialysis or ultrafiltration. In this method, primary grains obtained during the crystallization of the silver salt of an organic acid can be introduced into photosensitive layer without degrading the grains. However, problems of aggregation of grains under a condition of high salt concentration, an increase of viscosity upon concentration of the dispersion and so forth are not solved,;and thus this method is still insufficient as practical means for obtaining a uniform dispersion.
JP-A-9-127643 discloses a method of co-using a dispersing agent, similar to the method of JP-A-8-234358. However, the references contain no description of kinds of preferred dispersing agents. This method fails to provide superior dispersion stability, because the grain shape and the size are controlled in the presence of high salt concentration during generation of grains of silver salt of an organic acid and under co-existence of an organic solvent such as isopropyl alcohol.
In order to obtain fine and mono-dispersed grains of a silver salt of an organic acid, vigorous stirring is required during addition of an alkali metal salt solution and a solution containing silver ions. In particular, the solution of an organic acid alkali metal salt dissolved at a high temperature suffers from rapid temperature decrease to form precipitations upon addition thereof, and therefore large grains may grow if dilution rate or fluidization is slow or weak. When they are added to a tank in which a gas/liquid interface is present, and the stirring speed is increased, entrainment of air is caused. The grains of silver salt of organic acid are highly hydrophobic, and therefore the grains are adsorbed on the surfaces of the entrained bubbles to stabilize the bubbles and prevent breakage thereof, and in addition, the adjacent grains on the bubbles are aggregated. The liquid containing air entrained in such a manner becomes a whipped cream-like liquid of high viscosity, and disturbs uniform reaction.
Further, if the liquid temperature is still high after the reaction of the silver ion solution and the alkali metal salt solution, the grains grow due to physical ripening. Therefore, the liquid temperature is preferably maintained at around room temperature. Whilst, in order to obtain a stable solution of alkali metal salt of a long-chain fatty acid, it is necessary to maintain a high temperature of 50xc2x0 C. or more. Therefore, it is necessary to quickly obtain heat exchange for offsetting the heat introduced by the added solution. For example, in a method using a tank or the like provided with a jacket tank, when the concentration of dispersion produced from the reaction becomes high, fluidity of the liquid is disturbed and hence sufficient heat exchange cannot be achieved. Therefore, scaling up of the reaction and reaction in a concentrated liquid system are obstructed.
As described above, no method has been known so far which enables stable production of monodispersed silver salt of an organic acid with low fog, and independent control of the grain size and the grain shape.
An object of the present invention is to provide a method for producing silver salt of an organic acid, which solves the aforementioned problems. More specifically, the object is to provide a method for producing a silver salt of an organic acid, which can freely control the grain shape and grain size, and is suitable for scaling up or utilizing more concentrated solutions.
Another object of the present invention is to provide a photothermographic material having low fog and high sensitivity, and high blackening density, as well as having low haze and less image degradation after development, when it is used as a photothermographic material.
A further object of the present,invention is to provide a photothermographic material having good silver tone and photographic properties upon development and superior stability before and after light exposure and development.
The inventors of the present invention conducted extensive studies to achieve the aforementioned objects. As a result, they found that the aforementioned objects could be achieved by the characteristics mentioned below and achieved the present invention.
The present invention thus provides a method for producing a silver salt of an organic acid, which comprises the steps of reacting
(1) a solution containing silver ions in water or a mixture of an organic solvent and water, with
(2) a solution or a suspension containing an alkali metal salt of an organic acid in water, a mixture of an organic solvent and water, or an organic solvent to prepare a silver salt of an organic acid, and removing byproduct salt by a desalting operation, wherein a dispersing agent having a molecular weight of 3,000 or less is added and dispersed during the period ranging from before the reaction to before the desalting operation.
As preferred embodiments of the aforementioned method, there are provided the aforementioned method, which further comprises a step of adding a dispersing agent having a molecular weight of more than 3000 after completion of the reaction of the silver salt of an organic acid and before completion of the desalting operation; the aforementioned method, wherein the dispersing agent having a molecular weight of more than 3000 is a nonionic surfactant; the aforementioned method, wherein the desalting operation is carried out by ultrafiltration; the aforementioned method, wherein the dispersing agent having a molecular weight of 3000 or less is an ionic surfactant that has anionic nature and has a hydrophobic group having 8 to 40 carbon atoms; the aforementioned method, wherein the desalting operation is carried out by ultrafiltration, and a ultrafiltration membrane used has a fractional molecular weight 10 to 50 times as much as the molecular weight of the ionic surfactant; the aforementioned method, wherein the desalting operation is carried out by ultrafiltration, and a ultrafiltration membrane used provides 0 to 50% of rejection against the ionic surfactant; and the aforementioned method, wherein the dispersing agent having a molecular weight of more than 3000 has a molecular weight 5 to 50 times as much as the fractional molecular weight of the ultrafiltration membrane.
As one of preferred embodiments of the present invention, there is provided a method for producing a silver salt of an organic acid, wherein
(1) a solution containing silver ions in water or a mixture of an organic solvent and water, and
(2) a solution or suspension containing an alkali metal salt of an organic acid in water, a mixture of an organic solvent and water, or an organic solvent are reacted to form grains of silver salt of an organic acid,
(3) at least one surfactant having anionic nature and a hydrophobic group having 8 to 40 carbon atoms is added to a reaction mixture during a period ranging from before start of the reaction to before the desalting operation, and a macromolecular dispersing agent having a molecular weight 5 to 50 times as much as the fractional molecular weight of a ultrafiltration membrane used in the following step is added, and
(4) a byproduct salt is removed by ultrafiltration using a membrane having a fractional molecular weight 10 to 50 times as much as the molecular weight of the ionic surfactant, or (b) a byproduct salt is removed by ultrafiltration using a membrane achieving 0 to 50% of rejection against the ionic surfactant, or (c) a byproduct salt is removed by a combination of the operations of (a) and (b).
As preferred embodiments of the present invention, there are further provided the aforementioned method, wherein a concentration of the ionic surfactant is 5 to 100 times the critical micelle concentration of surfactant; the aforementioned method, wherein the ionic surfactant is added before completion of the addition of the solution containing silver ions; the aforementioned method, wherein a hydrophilic group of the ionic surfactant is sulfonic acid or a sulfuric acid ester salt having at least one aromatic group; the aforementioned method, which comprises a step of supplementing the ionic surfactant so:that the ionic surfactant concentration is kept constant during the desalting operation by ultrafiltration; the aforementioned method, which comprises a step of carrying out the desalting operation by ultrafiltration while supplementing a different ionic surfactant; the aforementioned method, wherein the ultrafiltration operation is performed prior to the addition of the nonionic macromolecular dispersing agent, andiconductivity of the organic acid silver salt dispersion is less than 2000xcexcS/cm when the nonionic macromolecular dispersing agent is added; the aforementioned method, wherein 2- to 10-fold constant volume dilution is performed after the addition of the nonionic macromolecular dispersing agent; the aforementioned method, wherein the concentration of the nonionic macromolecular dispersing agent is 0.1 to 30% by weight of the solid content of the silver salt of an organic acid; the aforementioned method, wherein the nonionic macromolecular is any one of polyvinyl alcohol, polyvinylpyrrolidone, or hydroxypropylcellulose, or a combination thereof; and the aforementioned method, which comprises a step of concentrating the dispersion to a concentration of 10% to 50% by weight after the constant volume dilution is performed.
In the presence of the ionic surfactant having anionic nature and a hydrophobic group having 8-40 carbon atoms, interfacial tension of the solid/liquid interface, selective adsorption to crystal surfaces of the silver salt of an organic acid and so forth can be controlled while the alkali metal salt of an organic acid and the silver ions are reacted to form grains of silver salt of an organic acid. Therefore, it becomes possible to selectively produce grains of any one of various shapes such as acicular shape, rod-like shape, scaly shape and tabular shape. In addition, the grain size and the grain size distribution can be readily controlled based on the aforementioned characteristics together with the defined mixing or temperature during the reaction. Furthermore, a dispersion obtained as described above will have a lower viscosity compared with a dispersion prepared without using the aforementioned surfactant, and therefore ease of stirring and mixing, heat exchange efficiency, handling property of the liquid and so forth are improved. Thus, the method is industrially advantageous as a production method having superior aptitude for manufacturing process and suitability for scaling up.
As the surfactant to be used, an anionic surfactant is preferred from viewpoints of solubility, adsorptivity to grains of silver salt of an organic acid and so forth. However, when an anionic surfactant is finally contained in a photothermographic material, hygroscopicity of the photosensitive material may become high due to its ionic nature, and it may sometimes adversely affect sensitivity and gradation, as well as color tone and image storability. In a preferred embodiment of the present invention, ad photothermographic material excellent in the total performance can be provided by replacing a low molecular weight dispersing agent, in particular, an ionic surfactant having anionic nature and a hydrophobic group having 8 to 40 carbon atoms, which adversely affects the performance of the photothermographic material although it is preferred for the formation of silver salt of an organic acid during the ultrafiltration operation, with a macromolecular dispersing agent of low hygroscopicity, in particular, a nonionic macromolecular dispersing agent.
As another aspect, there is provided a method for producing a photothermographic material containing a photosensitive silver halide, a silver salt of an organic acid, a reducing agent for silver ions and a binder on at least one side of a support, wherein the silver salt of an organic acid prepared by the aforementioned method is used. As a preferred embodiment of the method, there is provided the aforementioned method, wherein a binder of an image-forming layer containing the photosensitive silver halide and the silver salt of an organic acid is composed of a polymer having an equilibrium moisture content of 2% by weight or less at 25xc2x0 C. and 60% relative humidity, and the method comprises a step of coating the image-forming layer by using a coating solution containing 30% by weight or more of water in the solvent of the solution. Further, there is also provided a photothermographic material produced by the aforementioned method.
According to the method of the present invention, undesired byproduct inorganic salts or organic solvent used in the reaction can be removed without extracting the reacted silver salt of an organic acid as a solid and re-dispersing the same, and therefore the productivity is markedly improved. Further, by using the silver salt of an organic acid, there can be provided a photothermographic material having low fog, high sensitivity, and high density of blackening, as well as having low haze and less image degradation after storage. This photothermographic material is characterized in that it provides good silver color tone and photographic properties upon heat development, and the material is stable before and after the light exposure and development.